Enforce ES5 only

[rreusser]

It would be nice to include this as a test step. A quick script:

var CLIEngine = require("eslint").CLIEngine;

var cli = new CLIEngine({
    envs: ["browser", "mocha"], // ??? just copied it. not sure what this does.
    useEslintrc: false,
    ignore: false,
    parserOptions: {
        ecmaVersion: 5
    }   
});

var report = cli.executeOnFiles(["dist/plotly.js"]);
var formatter = cli.getFormatter();

console.log(formatter(report.results));

if (report.errorCount > 0) {
    process.exit(1);
}

Running this on 1.25.1 gives:

$ node es5only.js

/Users/rreusser/plotly/plotly.js/dist/plotly.js
  7:1  error  Parsing error: The keyword 'const' is reserved

✖ 1 problem (1 error, 0 warnings)

Remaining tasks:

1. find the right place for it (tasks?)
2. fix the ignore details. Right now it uses eslintignore which excludes everything we want to include. We either need to grab the directory listing and filter out /\.min\.js$ manually or figure out the right incantation. Should be simple.

[etpinard]

find the right place for it (tasks?)

https://github.com/plotly/plotly.js/blob/master/tasks/test_syntax.js

[rreusser]

Closing since it's all on the PR now.